In 1817, Sir Humphrey Davy published his theory on the cauMs of the illuminating property of flames. He stated tha/ this phenomenon was due to the presence in the midst; of/the flame of solid particles which, undergoing partial combustion only, were rendered incandescent. In a candle these were supposed to be solid particles of carbon. Recent experiments made by Dr. Edward Frankland seem ; to have entirely overthrown this theory. According to the researches of this eminent chemist many very brilliant flames exist in nature which cannot possibly contain any solid matter whatever. If, says he, metallk arsenic be burned in oxygen gas it produces a most intense white light. It is, however, well known that metallic arsenic is volatilized at a temperature of 180 C, and that the product of its combustion itself, arsen-lous acid, is also vaporized at 218 C, while the temperature Df incandescence of all solids has been proved to exceed 500 C, so that in this instance no solid particles could possibly exist in the flame. The vapor of sulphide of carbon burned in oxygen g-as or oxygen burned in sulphide of carbon produces a light so intense that the eye can scarcely bear it, and yet we are certain that no solid particles of matter are to be found here. The temperature of ebullition of sulphur, 440 C. is much below that of the flame produced in the above case. If protoxide of nitrogen be substituted for oxygen in this experiment the result is identical; the light created possessing sufficient intensity for the taking of instantaneous photographs or for producing the phenomena of fluorescence. Few bodies ignited, in oxygen gas emit a more powerful light than phosphorus. The product of this combustion is phosphoric acid, which is gaseous at a red heat, and which could not possibly have contained solid particles at the temperature of a flame which is capable of melting platinum. The conclusions arrived at by Dr. Frankland are that it is not solid particles which produce luminosity, but that the intensity of a flame depends on the radiation of dense but transparent hydrocarbon or other vapors. As a corollary to this theory he expresses his opinion, based on experimental researches, that a flame becomes luminous at a lower degree of temperature the denser the gases which enter into its composition, and he further infers that this luminosity is to a great extent independent of the nature of the vapor or gas, so that a gas which would burn without producing light at the pressure of the atmosphere, would become luminous, if submitted to a sufficient degree of compression. In order to prove these facts, Dr. Frankland caused the combustion of jets of hydrogen gas and of carbonic oxide gas to take place in oxygen gas under gradually increasing tensions up to twenty times that of the atmosphere. This he did in very strong iron vessels furnished with thick glass windows which allowed him to witness what occurred in their interior. Hydrogen gas, when burned in oxygen at the pressure of the atmosphere, gives a very feeble light. Under a pressure of two atmospheres this light is very noticeably increased, and at ten atmospheres a newspaper can be read at a distance of two feet without a reflector. Examined with the spectroscope the spectrum of this flame was bright, perfect, and continuous, from the red to the violet. The intensity of an electrical spark sent through a gaseous medium is also proportional to the density of the gases, being weak in hydrogen gas, greater in oxygen, and very considerable in chlorine, sulphurous acid gas, etc. A series of sparks from a powerful induction apparatus passing through air confined in a closed glass tube connected to a force pump, becomes brighter and brighter as the compression of the air is increased, and diminishes gradually in brilliancy as the air is allowed to escape. The electrical arch, produced by fifty couples of a Grove battery, is much increased when the vapor of mercury is allowed to intervene between the points of carbon of the electric lamp. The experiments of Frankland have elicited great attention from men of science, and a controversy is at present taking place on the subject at the Academy of Sciences, in Paris, where M. Sainte-Claire Deville affirms that H. Davy's theory is not subverted by the new discoveries, but that the facts observed may be satisfactorily explained if we admit, as lie believes to be the case, that the temperature of a flame is increased in the same ratio as the increased pressure or density under which the gas is ignited. The final verification of this physical law will need further elaborate and dangerous experiments, for the purpose of determining the temperature of combustion of various gases in oxygen, under various conditions of pressure higher than the atmospheric. These conclusive experiments will soon be begun in France at the.Ecole Normale, by order of the Emperor Napoleon III. The operators will be placed within a strong cylindrical iron chamber, where they will be surrounded by air, compressed to at least three times the weight of the atmosphere. Let us here remark that this pressure has been shown by the experiments of the bridge at Kehl to be harmless to the human organization. The results of these experiments may eventually have a very important practical bearing on the use of gas and of liquid fuels in our furnaces and under our boilers, the heating surfaces of which they may tend to diminish. They may also furnish us with an easy means of working platinum and of producing an indefinite amount of heat, and will probably be the means of suggesting some useful hints for the increase of the illuminating power of our ordinary lighting materials.
This article was originally published with the title "On the Theory of the Luminosity of Flames" in Scientific American 20, 15, 225-226 (April 1869)